Page 44 - Masonry and Concrete

Concrete Construction Techniques
43
CONCRETE CONSTRUCTION TECHNIQUES

using a convenient unit of measure such as a plastic bucket. If the con-
3
crete will be made with maximum 8-in. aggregate and air-entrained
1
cement, the correct proportions would be one bucket of cement, 2- 4
1
1
buckets of sand, 2 bucket of coarse aggregate, and 2 bucket of water.
The proportions remain the same relative to one another, regardless of
the size of the container used for batching. Depending on the amount
of moisture in the aggregates, the water content may have to be
adjusted slightly. Make a small trial batch to check the workability of
the mix and add more or less water if necessary.

3.2 Site Preparation

Site preparation will include a carefully measured layout of the size
and shape of the concrete, and excavating the existing soil or placing
structural fill to the required elevation.
3.2.1 Size and Layout

The layout for a floor slab or perimeter footings should be very precise
because it affects the layout of all the work which follows. Once you
have determined how to position a house on the site, roughly locate
each corner with wooden stakes, and then erect batter boards two to
three feet beyond the corners on each side (Figure 3-4). Use 2 4s for
the batter board stakes and 1 4s for the crosspieces. Drive the stakes
well into the ground and use braces if needed to secure them against
displacement from accidental bumps. If the site slopes, begin at the
highest corner of the building area and set the top of the first cross-
piece at 24 in. above the ground. Use a transit, a string-and-line level,
or a water level to mark the elevation on the other batter board stakes
and then set all of the crosspieces to the same reference elevation.
Place nails in the tops of the batter boards and run string lines to
mark the exact size and shape of the concrete (Figure 3-5). Using a
plumb bob, mark the intersection of the strings by driving a length of
steel-reinforcing bar into the ground to temporarily mark the exact cor-
ner of the concrete (Figure 3-6). Repeat this process at every inside and
outside corner, being sure to square each corner so that the dimensions
required by the drawings are exactly the same as those marked on the
ground. If the last dimension in the perimeter does not match the
drawings, then one or more of the corners is not square.

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